1. Field of the Invention
This invention relates to a steer-by-wire system having a fail-safe mechanism designed so that when a defect occurs in the steer-by-wire system, a shaft connected to a steering wheel is connected to a shaft that is connected to a steering gear, so as to make steering possible directly by the steering wheel.
2. Brief Description of the Prior Art
Though not commercially available at present, development of steer-by-wire systems in which the steering wheel is not mechanically connected to a steering gear is in progress in various places. This has merits, such as the capability of automatically controlling the wheel steering angle by car speed detection and increasing the degree of freedom of car interior layout, and is expected to enable more stabilized vehicle running and to improve driving performance.
An example of such steer-by-wire systems will be described with reference to FIG. 10. A steer-by-wire system 100, as shown in FIG. 10, comprises a steering wheel 110, a shaft 120 connected to the steering wheel 110, a steering gear 130, a shaft 140 connected to the steering gear 130, a reaction simulator 150 mounted on said shaft 120, a steering actuator 160 mounted on the shaft 140, a backup clutch 170 mounted between the shafts 120 and 140, and a controller 180 for controlling the reaction simulator 150, steering actuator 160, and clutch 170.
This steer-by-wire system 100 is arranged to timely cancel the connection between the steering wheel 110 and the steering gear 130 established by the clutch 170 so as to control the steering angle of the steering gear 130 by the controller 180 through the steering actuator 160 according to the steering angle of the steering wheel 110, while making the steering reaction controllable by the reaction simulator 150.
In such steer-by-wire, however, if a defect should occur in the motor or the like (for example, the steering actuator 160 or reaction simulator 150), a defect sometimes occurs in steering-based vehicle control. As a fail-safe mechanism, it has been proposed to use a solenoid clutch as the clutch 170 to interconnect the steering wheel and the steering gear so as to enable steering directly by the steering wheel. See, for example, Japanese Unexamined Patent Publication 2001-301639.
A solenoid clutch type steer-by-wire 200 described in the Patent Publication, as shown in FIG. 11, comprises a steering wheel 210, a shaft 220 connected to the steering wheel 210, a steering gear 230, which is a steering wheel, a shaft 240 connected to the steering gear 230, a solenoid clutch 250 mounted on the shaft 220, a solenoid clutch 260 mounted on the shaft 240, a shaft 270 connecting these solenoid clutches 250 and 260, a gear 280 mounted on the shaft 270, a motor 290 for driving the gear 280, a gear 300 mounted on the shaft 240, a motor 310 for driving the gear 300, a steering angle sensor 320 mounted on the shaft 220, a controller 330 for controlling the solenoid clutches 250 and 260 and motors 290 and 310 by the motors 290 and 310 and a input signal from the steering angle sensor 320, and a car speed sensor 340 for detecting the car speed to input the latter into the controller 330.
The steer-by-wire system 200 described in the Patent Publication mentioned above uses solenoid clutches 250 and 260, so that if the power supply is interrupted by a malfunction of the electrical system or the like, switching operation of the solenoid clutches 250 and 260 cannot be effected, making direct connection between the steering wheel 210 and the steering gear 230 impossible. Further, the use of solenoid clutches has many drawbacks, such as the necessity of securing electric power supply capacity to some extent and the necessity of enlarging the installation space.